The present invention concerns spinal fixation systems, and particularly systems utilizing elongated rods adjacent the spinous process providing a base for connecting fixation elements to several vertebral levels. More specifically, the invention concerns improvements to the manner in which the vertebral fixation elements are engaged to the elongated spinal rod.
Several techniques and systems have been developed for correcting and stabilizing spinal curves and facilitating spinal fusion. In one system, a bendable rod is longitudinally disposed adjacent the vertebral column, or spinous process, and is fixed to various vertebrae along the length of the column by way of a number of fixation elements. A variety of fixation elements can be provided which are configured to engage specific portions of the vertebra. For instance, one such fixation element is a spinal compression/distraction hook. This type of spinal hook is used to anchor the rod by engaging the laminae of the vertebra. Another fixation element is a spinal screw, which includes cancellous threads for engagement within the pedicle of a vertebra.
An example of a rod-type spinal fixation system under consideration with the present invention is the TSRH.RTM. spinal system sold by Danek Medical, Inc. In this system, a spinal hook, such as the hook H shown in FIG. 1 for example, is engaged to an elongated fixation rod R by way of an eyebolt assembly E. As is known in the art, the eyebolt E is mounted on the spinal rod and captured within yokes on the spinal hook. A nut N is then threaded onto a threaded post of the eyebolt to clamp the hook yoke between the nut and the fixation rod R. In this manner, the eyebolt E and the yokes of the hook H provide three degrees of fixation as represented by the arrows in FIG. 1. Details of the TSRH.RTM. spinal implant system are disclosed in the "TSRH.RTM. Surgical Technique Manual provided by Danek Medical, Inc., published in 1990, which disclosure is incorporated herein by reference.
It is the goal of the surgeon using spinal implant systems such as the Danek TSRH.RTM. system to apply vertebral fixation elements, such as a spinal hook or a bone screw, to the spine in appropriate anatomic positions, and then to engage each fixation element to the spinal rod. One problem with the spinal hooks H of the prior art, as represented in FIGS. 1 and 2, is that the hooks are rather bulky and wide since the fixation yokes of the hook were configured to surround the spinal rod R. Moreover it had been found that hooks such as hook H only allow the rod to be implanted in one position relative to the spinal column as dictated by the required position of the shoe of the hook against the vertebra.
In order to address that and other problems with the prior art systems shown in FIGS. 1 and 2, new spinal fixation elements have been developed which are the subject of pending applications assigned to the assignee of the present invention. One such fixation element, a spinal hook 10, is shown in FIGS. 3 and 4, includes a shoe 11 having a bone engaging surface 12. The bone engaging surface 12 can be formed in any known shape to engage a laminae of a vertebra, for instance. Integral with the shoe 11 is a top portion 15 that forms a pair of posts 14 disposed apart from each other in the form of a U-shaped yoke to define a slot 15 therebetween. The slot 15 is wide enough to receive an eyebolt assembly, such as eyebolt assembly E shown in FIGS. 1 and 2. A pair of coaxial grooves 16 formed in each lateral surface 18 of the hook 10 are each configured to receive a portion of a spinal rod, such as rod R shown in FIGS. 1 and 2. These rod grooves 16 are present on each lateral surface 18 of the posts 14 so that the hook 10 can be oriented on either side of a spinal rod. Slots 17 are provided on each end face 19 for engagement by a hook-holding insertion instrument.
Another newly developed vertebral fixation element is a variable angle spinal screw, which is the subject of a pending application assigned to the assignee of the present invention. Certain details of this novel variable angle screw system is shown in FIG. 10. In particular, the screw 70 includes a shank 71 having bone engaging or cancellous threads formed thereon. The screw 70 also includes an upper yoke 73 formed by a pair of posts which define a U-shaped slot opening 74 configured to engage a specially designed connector for connecting the screw 70 to a spinal rod. The screw 70 includes an interdigitating face 76 having a number of radial splines 77 formed thereon.
Leaving the details of these newly developed vertebral fixation elements to their respective pending applications, it can be appreciated that the central post hook 10, of FIGS. 3 and 4, and the variable angle screw 70, of FIG. 10, have increased the versatility of rod-type spinal implant systems, such as the TRSH.RTM. system provided by Danek Medical. However, one feature consistent between these newly developed fixation systems, as well as the prior art spinal hook H shown in FIG. 1, is that the eyebolt assembly E used to engage the fixation component to the spinal rod is "side-tightening". In other words, the threaded post of the eyebolt E and the nut N engaging the post both project laterally away from the spinal rod R, as specifically depicted in FIGS. 1 and 2. It has been found in practice that it is often cumbersome to get engage the nut N with a wrench to tighten the nut onto the eyebolt assembly E. Moreover, simple mechanics dictates that the wrench can only be moved through a partial turn before the handle of the wrench contacts the surrounding tissue. This necessitates taking the wrench off of the nut and re-engaging it for an additional partial rotation. Ratchet type wrench systems are typically not acceptable in procedures of this sort because the lateral space required for the ratchet mechanism unnecessarily impinges on the surrounding tissue and requires greater space at the surgical site.
Spinal and orthopaedic procedures are rapidly becoming prevalent surgeries, largely because of the high incidence of low back pain syndrome. In the past, surgical techniques for alleviating low back pain or for addressing spinal deformities or injuries has required fairly complicated and massive surgical techniques. The focus in recent times has been to greatly reduce the degree of invasion into the patient required for instrumenting a spine, as well as to reduce the amount of trauma to tissue surrounding the instrumentation, both during the procedure and after the spinal instrumentation has been implanted.
One cog in this worthwhile goal for minimally invasive spinal surgical techniques, is to provide an improved means for clamping the various vertebral fixation components to a spinal rod in situ. Such a system should eliminate the side-tightening requirement of prior art systems. In addition, such a system should retain the versatility achieved by newly developed central post hooks and variable angle screw fixation elements. It is the goal of the present invention to address this and other concerns.